The invention relates generally to immersion lithography apparatus and methods, and particularly to apparatus and methods for recovering droplets of immersion liquid that have been left on a substrate during immersion lithography.
Lithography exposure apparatus are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle containing one or more patterns, a projection system, a wafer stage assembly that positions a semiconductor wafer, and a measurement system that precisely monitors the positions of the reticle and the wafer. As is known, lithography exposure apparatus also can be used to form images on substrates other than semiconductor wafers, for example, glass or quartz substrates in order to form, for example, flat panel displays such as LCD displays.
Immersion lithography is a technique that can enhance the resolution of lithography exposure apparatus by permitting exposure to take place with a numerical aperture (NA) that is greater than the NA that can be achieved in conventional “dry” lithography exposure apparatus. By filling the space between the final optical element of the projection system and the resist-coated target (wafer or other substrate) with immersion fluid, immersion lithography permits exposure with light that would otherwise be internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion fluid (or of the resist or lens material, whichever is least) are possible in immersion lithography systems. Liquid immersion also increases the wafer depth-of-focus, that is, the tolerable error in the vertical position of the wafer, by the index of the immersion fluid compared to a dry system having the same numerical aperture. Immersion lithography thus has the potential to improve resolution enhancement equivalent to a shift from 193 nm to 157 nm without actually decreasing the exposure light wavelength. Thus, unlike a shift in the exposure light wavelength, the use of immersion would not require the development of new light sources, optical materials (for the illumination and projection systems) or coatings, and should allow the use of the same or similar resists as conventional “dry” lithography at the same wavelength. In an immersion system in which only the final optical element of the projection system and its housing and the wafer (and perhaps portions of the stage as well) are in contact with the immersion fluid, much of the technology and design developed for dry lithography can carry over directly to immersion lithography.
However, because the wafer (or other substrate) moves rapidly in a typical lithography system, some of the immersion fluid in the immersion area between the projection system and the substrate tends to be carried away from the immersion area. If the immersion fluid escapes from the immersion area, that fluid can interfere with operation of other components of the lithography system. One way to prevent the immersion fluid from carrying away from the immersion area is described in WO 2005/24517, which corresponds to U.S. patent application Ser. No. 11/362,833, the disclosure of which is incorporated herein by reference in its entirety. This document discloses an immersion fluid supply and recovery system in which a porous member surrounds the immersion area and is in fluid communication with the space that defines the immersion area. The porous member is maintained at a pressure that is under the bubble point of the porous member, whereby immersion fluid that escapes from the immersion area is captured (recovered) by the porous member.
Other fluid recovery and fluid containment systems have been described. For example, in one such system described in US-2004/0165159-A1, the immersion liquid is maintained in the immersion area by forming an “air curtain” around the immersion area by supplying pressurized gas to the area surrounding the immersion area.
However, even when there is a satisfactory fluid recovery/containment system, droplets of immersion liquid can be left on portions of the substrate after the portion of the substrate has passed beneath the projection system and the underlying immersion area. Such droplets cause local cooling of the wafer and the environment around the wafer when the droplets evaporate, which can have an undesirable effect on the exposure operation. For example, evaporation of the droplets can change the refractive index in the optical path of the detecting light used by the lithography apparatus interferometer(s), and thus adversely affect the measurements made during the exposure operation. Evaporation of the droplets also can leave an area of contamination on the wafer. Furthermore, subsequent high acceleration movements of the substrate can cause droplets to be scattered to surrounding components of the lithography apparatus, causing those components to corrode or at least fouling those components.
WO 2005/020299 and its related US 2006/0139593 A1 disclose an immersion lithography apparatus that moves the substrate relative to an immersion fluid supply/recovery nozzle after the entire substrate has been exposed in order to remove any immersion fluid that may remain on the substrate.